This invention relates to gas-insulated transmission systems, and more specifically relates to a novel elastomeric seal for preventing contamination products from reaching the interior of a gas-insulated transmission system during the welding of the outer housings.
Gas-insulated transmission systems of the type to which the invention applies are shown in the above-noted U.S. Pat. No. 3,813,475 and are generally comprised of an elongated conductor which is supported centrally within a cylindrical grounded housing by means of spaced insulator support devices. The interior of the outer grounded conductive cylinder is then filled with a suitable gas having desired dielectric characteristics, such as sulfur hexafluoride under pressure. The bus is then operated at extremely high voltages, for example, 500 KV. The outer housing of the buses have relatively small diameters, for example, 15 inches. This creates exceptionally high electric fields within the interior of the gas-filled housing so that great caution must be exercised to prevent contamination including moisture, conductive particles, dust and the like from reaching the interior of the bus since the presence of these contaminants can lead to dielectric failure of the bus. Thus, the components which constitute the transmission line must be clean and dry and the gas which fills the housing must also be clean and dry and the housing interior must be free of conductive particles and the like. Conductive particle traps are commonly used along the length of the transmission line and are formed by low field intensity regions which will tend to trap conductive contaminants once these contaminants reach the trap region.
One serious source of contamination of the interior of the grounded housing is the welding process in which outer enclosure sections are welded to one another during the installation of the line. That is, the transmission line is frequently made in discrete lengths which are factory-assembled and then shipped to a site for installation. These factory-assembled sections, which might each be 50 feet long, must then be connected together in the field, and the outer enclosures are commonly welded to one another to form a gas-tight seal and to cause the housing sections to become electrically continuous. During this welding process, weld spatter and other contamination can enter the interior of the housings and can interfere with the electrical performance of the system.
In the past and in order to prevent contamination of the interior of the conductive housing during welding, various attempts have been made to seal the joint before welding as by wrapping insulation tape on the inner member of a telescoping joint; through the use of wiper rings; through the use of solid "O" rings; and through the use of various geometries for the shoulders of the backup welding rings. All of these prior methods had one or more disadvantages. Thus, none were able to accommodate large tolerances and out-of-roundness variations of the sections to be joined. Moreover, some were expensive, as the use of solid O rings, wiper rings or high temperature tape in the joint. The mere use of extended shoulders for the backup rings has the disadvantage in that it is not a positive method for contamination control.